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II                                 DEVELOPMENT OF OVA                               7 1

Eutherian mammals, including the Marsupials, are very small as
compared with those of any other vertebrates, excepting only
^Amp7iioxi(.s, where the young are hatched early as free swimming
larvae. They also differ in a highly characteristic way in the
mode of their development within the ovary. These processes
are to some extent illustrated in Fig. 44. The main frame-
work of the ovary is formed of the so-called *' strorna," which is
a mass of tissue formed of more or less connective-tissue-like
cells. Within this are numerous cavities, the Graafian follicles.
The very young follicles consist of but a single layer of follicular
cells surrounding the ovum, which lies centrally. The follicular

Fra. 45.—Two stages in the development of the Graafian follicle. A, With, the folli-
cular fluid beginning to appear ; B, after the space has largely increased. capsa
Capsule; disc, cumulus proligerus; memb, membrana granulosa ; ov, ovum ; s%>>
space containing fluid. (After Hertwig.)
cells gradually increase in number until the ovum lies in the
midst of several layers of cells. At this period a vacuity is
formed between some of these cells, and grow., into a large
cell-free cavity; the ovum does not lie loosely in this space, but
is connected at one side with the follicular cells, which still line
the interior of the Graafian follicle by the so-called discus
or cumulus proligerus. The egg or ovum has, moreover, a layer
of cells immediately surrounding itself. All these facts can be
gathered by an inspection of Fig. 45. It has been shown
that, as in lower vertebrates, the cells immediately surrounding
the ovum are connected with it directly by delicate processes
which penetrate the actual membrane of the egg.
The only ova which depart at all in. structure from that above
described  are   those   of the  Monotremata.     The  credit   of  this